Cathode for thermionic devices and method of producing same



June 10, 1930. LA. 'MCLI. NlcoLsoN 1,762,531

CATHODE FOR THERMIONI C DEVICES AND METHOD OF PRODUCING SAME Filed Feb.14, 1927' IN V EN TOR.

A TTORNEX Patented June 10, 1930 m oFFica ALEXANDER MCLEAN NIOOLSON, OFYORK, N. Y., ASSIGNOB TO COMMUNICATION PATENTS, INCL, OF NEW YORK, N.Y., A CORPORATION OF DELAWARE .CA'lI-IODE FOR THEBMIONIC DEVICES ANDMETHOD OF PRODUCING SAME Application filed February 14, 1927. Serial No.168,242.

This invention relates to electron emitting electrodes, employed inaudions, vacuum tubes, and thermionic devices, and particularly to thosein which the emission of electrons results from anemissive surfaceformedby the application of a coating to a base orcore, and to methods ofpreparing the same.

It has for an object the provision of an electron emitting electrodehaving a comparatively thin and regular coating or emissive surface ofsubstantially uniform depth and in which the electronically inactive andphysically unstable portions have been reduced to a minimum.

Another object is the provision of an electron emitting electrodesurface whose thickness and therefore capacity for emission or activitymay be regulated during the process of production and increased asdesired while remaining closely adherent in itself and to the core andwhich coating is substantially entirely active at comparatively lowoperating temperatures.

Another object is the provision of a method of decreasing and reducingto a minimum the quantity of substantially electronically inactive andmechanically or physically unstable components in the emissive coatingof a completed filament or electrode.

Another object is the provision of a method of decreasing and reducingto a minimum the quantity of substantially electronically inactive andmechanically or physically unstable components in the emissivc coatingof a completed filament or electrode.

Another object is the provision of a method of producing electronemissive coatings on a conductive base or core in which the surfaceproduced is regular, substantially entirely active and conductive, andmay be made of any desired thickness with out becoming non-adherent.

Generally stated, the accomplishment of the above objects is based'uponmy discovery that the surface formed by coating a base or core is madeup of two components, one of which is essentially the source of theelectron-emitting activity of the filament While the other issubstantially non-essential as regards the emissive characteristics ofthe filaments. This essential component,

I have found, comprises substantially a thin I film formed adjacent theconductor and adhering particularly well to the latter. I believe thisfilm to be essentially a chemical. combination, as alloyed or compoundedwith the composition of the core, but regardless of this phase of thematter it may be said to be closely attached to or associated with thecore.- Working upon this basis, I have achieved theabove objects bytreating the coated conductor or core after the application of a coatingcomposition so as to remove the physically unstable and chemicallyuncombined constituents from the surface.

More particularly this treatment according to my invention comprises,the subjecting of the coated and baked conductor to a dissolvingprocess, such as a leeching in water or in an alkaline solution, wherebythe readily soluble and physically unstable components are separatedfrom the emissive surface, leaving the active portion thereon. In thismanner any desired depth of completed emissive surface which is closelyadherent may be procured by repeatingthe successive steps of coating,baking and leeching a sufficient number of times. The surface remainingon the conductor after this leeching treatment may be termedsubstantially the under-coat of the originally applied coating and isremarkably superior to the original coating formed merely by applicationand baking.

To these and other ends, the invention consists in further features allof which will be more fully described and thoroughly understood with theaid of the description to follow, the novel features being particularlyset forth in the appended claims.

In the drawing, Figure 1 shows in greatly magnified form the relativethicknesses of the undercoat or adhering portion 3 and the overcoat orunstableportion 4 of a thermionic'coating applied to a fiat ribbonfilamentary core 1, the latter having been found suitable for use as anelectron emitting cathode.

employing a heated filament, as exemplified by the three electrodevacuum tubes containing a filament, and input and output electrodes, andwhose operating characteristics are directly dependent upon thecomposition or substance used as the filament or cathode. Of the factorsinfluencing the choice of a cathode, the electron aflinity (ratio ofelectron evaporation constant to the electron charge) is probably themost important and for general uses of thermionic devices, this factoris maintained as small as possible since the power dissipated in heatingthe cathode to obtain a definite thermionic current decreases as theelectron afiinity is decreased. Thus, a low value of the electrodeaflinity constant implies economy of operation and increased lifepf bothfilament and tube, because of the lower temperature at which thefilament or cathode can be operated to produce the desired saturationcurrent values. To this end, the cathode is preferably treated to obtaina low value of the electron aflinity and one of such treatments is theWehnelt process which comprises coating a conductive and highly re-'fractory core, as platinum, with the oxide of one of the alkaline earthmetals, these latter compositions having been found to possess highemissivity or activity characteristics, or in other words low electronaflinity values.

Another filament characteristic which is determinative in the choice ofa filament which will operate at low temperatures producing suitableactivity values is the heat characteristic of the emissiveconductivitsurface whlch should beas high as possible, since the surface. derivesits heat substantially entirely by radiation and diffusion from theconductive heated core and the heat conductivity of the surface willthus determine the temperature to which the'core must be heated toproduce a given temperature in the coating.

In accordance with my invention, I have found that the emissive oractivity characteristics of coated filaments are dueessentially to athin almost imponderable undercoat film 3 formed immediately adjacentthe conducting core while the remainder 4 of the coated surface, whichfor the purpose of description may be termed the overcoat, does notexert any material beneficial influence upon the o eration of thefilament On the other han this overcoat portion possesses anundesirablehigh resistance thereby causing a loss of power from unnecessary heat-,ing and further possesses an irregular surface. while the undercoat filmis comparadiscovery, my improved process includes a treatment of thecoated and baked conductor whereby the overcoat portion is removed andthe undercoat film unafiected, leaving essen-. tially only the thinundercoat film, and the steps of the coating, baking and overcoatremoval treatment are repeated until a finishedcoating surface of thedesired thickness has been built up. By this process, I

am .able to produce a thermionically active 7 filament having an activesurface of uniform depth thus appreciably reducing the likelihood ofoccurrence of bright spots in the-heated filament, and possessing aregular even surface which closely approximates the desired regularityof an uncoated conductor surface. Furthermore, the desirable strongadherence of the coating in itself and to the core is not reduced byincreasing the thickness of the surface and successively appliedundercoat films adhere strongly to each other, thus making possible theproduction of any desired thickness of surface or degree of activity.The life of the filament is also materially increased, since the entirecoating is highly electrically conductive, thereby implying lowtemperature of. operation.

In the preferred treatment, the conductor after being coated and bakedis passed through a dissolving or leeching treatment in a solvent whichwill dissolve the overcoat portion without materially affecting theundercoat film. Solvents which have been. found satisfactory, includewater and various alkaline solutions, as sodium hydroxide, potassiumhydroxide and ammonium hydroxide. The duration of leeching treatmentwill of cours'e depend upon the amount of coating applied andupon thestrength and temperature of the solvent, but as a general rule I havefound that the greater numlength and 0.615 grams weight which had beengiven a coated and baked surface of 0.035 grams Weight and found firstthat after five minutes leeching there remained an undercoat filmweighting 0.005 grams and second that a continuous leeching of the samecoated and baked filament for as long as fifty hours resulted in nofurther removal of the coating.

Various materials may be used as the actual coating composition, but Ihave found the use of a mixture of a highly active material with abinder material, as disclosed in 'my copending applications, Serial No.108,676, filed May 12th, 1926; No. 108,961, filed May 13th, 1926; No.154,035, filed December 10th, 1926; and in my joint patent with E. C.Hull, No. 1,209,324, dated December 19th, 1916. Briefly thesedisclosures comprise the use of an emissive surface including both analkaline-earth material, particularly a compound of barium, and amaterial which may or may not possess any activity characteristics butwhich serves as a binder between the active alkaline earth material andthe material of the core. The binder materials disclosed and claimed, asabove listed, comprise compounds of strontium, the rare earth group(titanium, zirconium, cerium, thorium, lanthanum, praseodymium,samarium, yttrium, europium, gadolinium, terbium, dysprosium, holmium,erbium,thulium, ytterbium, lutecium,) and the group contained in thefourth series of the periodictable between groups three and eightinclusive (scandium, titanium, uranium, chromium, manganese, iron,cobalt, nickel), the oxides of these metals being preferable. When suchmaterials are employed as the .coating compositions, I have found thatthe undercoat film is. particularly adherent and believe that this filmor layer is composed of the compounds of the coating metals used withthe core material employed.

Noble metal cores, .and particularly those composed essentially ofplatinum as platinum and platinum-iridium alloys, are especiallysatisfactory for use in accordance with my invention.

Myimproved process is especially advantageous in that the activity oremissivity properties ofthe coated filament may be inleeching asufficientnumber of times, since the removal of the inactive overcoatportions makes it possible to bring fresh material into contact with thecore and the conductive undercoat film to thereby form a fresh layer ofthe latter. Without the re-' moval of the overcoat, it would besubstantially impossible to increase the thickness of the essentialundercoat film, as there could then be no interaction between the corecomposition and the uncombined portions of th applied coatingcomposition.

What I claim is 1. The method of producing an electron emittingelectrode which comprises coating a conducting core with an electronemissive material, baking the coated core, andremoving the uncombinedcoating material.

2. The method of forming an electron emissive electrode which comprisesrepeating the successive operations of coating an electricallyconductive core with an electron emissive. material, baking the coatedcore and removing the chemically uncombined coating material to-producea coating surface of the desired thickness.

3.. The method of producing a thermionically active filament whichincludes the step of removing the uncombined portions of the coating.

4. The method of'producing a coated electron emissive filament whichincludes the step of leeching the coated conductor to remove theuncombined constituents.

5. The method of producing a coated electron emissive filament whichincludes the steps of coating a conductive core with an electronemissive material, baking the material thereon, and leeching the bakedconductor to remove the overcoat portion.

6. The method of making an electron emitting cathode, Which comprisescoatinga conductive core with a composition containing an alkalineearth-metal compound, baking said coated conductor, to combine a portionof the coating with the core composition, and subjecting the cathodeelement to a dissolving treatment to remove the uncombined portion ofthe "coating.

This specification signed this 9th day of February, 1926.

ALEXANDER McLEAN NICOLSON.

creased to any desired value by repeating v

